Power converters such as switch-mode voltage regulators are widely used in various electronic devices for sourcing power to the electronic devices from a power source. Taking a buck type switching regulator for example, the buck switching regulator generally has relatively high conversion efficiency, wide bandwidth and good loop stability with sample loop compensation, and is thus popular in converting high input voltage to relatively low output voltage applications. FIG. 1 illustrates schematically a typical buck type DC-DC voltage converter 100. In brief, the voltage converter 100 is configured to receive an input voltage Vin at its input terminal IN, and convert the input voltage Vin into an appropriate output voltage Vo through controlling a high side switch MH and a low side switch ML to switch on and off in a complementary manner. The voltage converter 100 comprises a control circuit 101 for providing control signals to the high side switch MH and the low side switch ML. The high side switch MH and the low side switch ML are connected in series between the input terminal IN and reference ground GND, the common connection SW (also referred to as switching voltage output node SW) is coupled to an output terminal OUT of the voltage converter 100 via an inductive energy storage component LO. A capacitive energy storage component CO is coupled between the output terminal OUT and the reference ground GND to smooth the output voltage VO.
The high side switch MH may comprise an N-channel power switching device, such as an N-channel FET or an N-channel DMOS etc. to save chip area, reduce the size and improve the performance of the voltage converter 100. In this situation, in order to make the high side switch MH to be fully turned on (i.e. to make the high side switch MH to operate in saturation region in which the switch MH has a quite small on resistance), a voltage applied between a control terminal and a terminal connected to the node SW of the high side switch MH must be large enough, at least larger than a turn on threshold voltage of the high switch MH. For instance, in the example where the high side switch is a FET/DMOS, the voltage between a gate terminal and a source terminal (connected to the node SW) of the FET/DMOS must be larger than a turn on threshold of the FET/DMOS. However, when the high side switch MH is on, the voltage at the node SW can reach the input voltage Vin, and thus a voltage higher than the input voltage Vin must be provided to the control terminal of the high side switch MH so as to turn it on completely.
Therefore, in order to generate a voltage higher than the input voltage Vin, the voltage converter 100 generally further comprises a bootstrap circuit 102. The bootstrap circuit 102 is configured to provide a bootstrap voltage VBS referenced to the voltage at the node SW. The bootstrap voltage VBS can be used to enhance the driving capability of the control signal DH provided to the control terminal of the high side switch MH, so that the control signal DH can drive the high side switch MH to turn on and off in good condition. In the example of FIG. 1, the bootstrap circuit 102 and a bootstrap capacitor CB are connected in series between the input voltage Vin and the switching voltage output node SW, the bootstrap circuit 102 comprises a first reference REG1 and a diode DB, wherein a cathode of the diode DB is connected to the bootstrap supply terminal VB, an anode of the diode DB is connected to a first terminal of the bootstrap capacitor CB, and a second terminal of the bootstrap capacitor CB is connected to the node SW. A voltage across the capacitor CB is provided as the bootstrap voltage VBS. The operating principles of the bootstrap circuit 102 can be easily understood by the ordinary artisan. When the high side switch MH is turned off and the low side switch ML is turned on, the bootstrap capacitor CB is charged by the bootstrap supply voltage till the voltage across the bootstrap capacitor CB reaches the bootstrap voltage VBS. When the high side switch MH is turned on and the low side switch ML is turned off, the input voltage Vin of the voltage converter 100 is transmitted to the switching voltage output node SW, i.e. the voltage at the second terminal of the bootstrap capacitor CB is pulled up to the input voltage Vin. Thus, the voltage at the first terminal of the bootstrap capacitor CB is raised to a voltage higher than the input voltage Vin, substantially equals to the input voltage Vin superposing the bootstrap voltage VBS. As the voltage at the first terminal of the bootstrap capacitor CB reaches to the input voltage Vin plus the bootstrap voltage VBS, the diode DB is reversely biased and is thus turned off so as to protect the bootstrap supply voltage source from being damaged by the relatively higher input voltage Vin.
In view of the above, it can be understood that the bootstrap capacitor CB can not be charged/recharged to refresh the bootstrap voltage VBS unless the low side switch ML is turned on. However, in certain circumstances, the bootstrap capacitor CB may not have enough charge stored and may not be charged/recharged in time, resulting in the bootstrap voltage VBS to be decreased, which may cause the control signal DH to not be able to drive the high side switch MH to turn on and off properly. In such a situation, the voltage converter 100 will no longer be able to operate normally, which is not desired. For example, when the voltage converter 100 operates in light load or no load condition, the control circuit 101 is configured to reduce the on time and/or the switching frequency of the high side switch MH and the low side switch ML to improve the conversion efficiency of the converter 100. However, this may lead to the capacitor CB not being able to be charged/recharged in time because the on time of the low side switch ML is too short or the high side switch MH and the low side switch ML do not switch in a relatively long time. In other circumstance, for example, if the desired value of the output voltage Vo is close to the input voltage Vin, the high side switch MH has to operate in quite high duty cycle or 100% duty cycle, wherein the duty cycle refers to a percentage of the on time of the high side switch MH in the switching cycle of the switches MH and ML. In this case, the on time of the low side switch ML in one switching cycle may be quite short or the low side switch ML may even have no chance to turn on, resulting that the bootstrap capacitor CB can not be charged/recharged in time to store enough electrical charges to provide a high enough bootstrap voltage VBS. The bootstrap capacitor CB should wait till the output voltage Vo drops, which implies that the duty cycle decreases and the on time of the low side switch ML increases, in order to be charged/recharged so as to refresh the bootstrap voltage VBS (i.e. to make the bootstrap voltage VBS restore to a high enough value). However, this event can result in large spikes in the output voltage Vo. For example, supposing the converter 100 has a 6V input voltage Vin and a 3.3V desired output voltage Vo, a 3V bootstrap voltage VBS is required to ensure the high side switch MH to be turned on and off normally. In this example, if an output current drawn from the converter 100 by a load is relatively small or no output current is drawn (i.e. the converter 100 operates in light load or no load condition), the bootstrap voltage VBS will decrease to smaller than 2.7V, inducing the high side switch MH not being able to turn on normally. The bootstrap capacitor CB should wait until the output voltage VO decreases to smaller than 3V in order to be recharged so that the bootstrap voltage VBS can refresh/restore to 3V. Then, the high side switch MH and the low side switch ML can switch normally to regulate the output voltage Vo to resume to its desired value 3.3V. However, each time the output voltage Vo resumes from smaller than 3V to 3.3V, a large spike occurs, which is harmful to the converter 100 and the load, and thus is undesirable.
A need therefore exists for solving the problem of charging the bootstrap capacitor CB in power converters.